Fiber-optic rotation sensors or gyros, as they are commonly called, are increasingly being used for detection of rotation, particularly in navigation systems such as those used in motor vehicles, aircraft, and spacecraft, where accurate and reliable sensing of inertial rotation is highly critical.
In a typical fiber-optic gyro, light from a laser or some other suitable light source is divided into two separate beams by means of a beam splitter and then coupled into the two ends of a multi-turn coil of optical fiber, typically of the single-mode type. Light emerging from the two fiber ends is combined by the beam splitter and detected by a photodetector.
Rotation sensing is typically accomplished by detection of a rotationally induced phase shift, commonly referred to as the "Sagnac phase shift," between the light beams propagating in opposite directions around the coil of optical fiber. The detected signal corresponding to the phase difference between the counter-propagating beams is typically subjected to some form of phase modulation, and the photodetector converts the modulation to an electric signal which is indicative of the degree of rotation of the fiber coil and is electronically processed to provide a direct indication thereof.
It follows that any other effect which causes a non-reciprocal difference in phase between the two paths during the transit time of the light around the coil will be registered as a false rotation. One such effect, the Shupe effect, is due to the passing of a thermal gradient through the coil; the change in temperature produces a change in the refractive index of the material from which the fiber is made. Typically for silica the change of index with temperature is approximately 10 parts per million per .degree.C. The effect is best illustrated by considering a gyro with a coil having a single turn.
If, as in FIG. 1, a thermal gradient traverses the coil so that it is symmetrical with respect to the mid-point M of the fiber that forms the coil, sections on either side of M will experience identical changes in index at any time so that there will be no net phase difference between clockwise and counterclockwise light paths. However, if, as in FIG. 2, the thermal gradient traverses the coil asymmetrically with respect to the mid-point, the sections on either side of M will experience different changes in index at different times producing a change in phase between the two light paths and thus giving a spurious rotation signal. The same situation holds for a multi-turn coil. Here there is a phase shift between the clockwise and counterclockwise paths if there is any net asymmetry of thermal gradient integrated over all the turns of the coil.
The Shupe effect generally increases as the length of fiber used to form the coil decreases, because fewer turns are formed as the fiber length is reduced.
It is current practice to reduce the Shupe effect by winding the coil in a controlled and ordered manner such that the fiber elements equidistant from the midpoint are adjacent so that the temperature gradient at all points affects the fiber on both sides of the midpoint equally and at the same time. Such coils, known as Shupe coils, involve a very complicated and controlled winding procedure and are consequently expensive to produce.
Another problem with fiber-optic gyros is the need to maintain the axis of the sensing coil perpendicular to the plane in which the inertial rotation is being measured. If this perpendicular relationship is not maintained, then the rotation sensed by the gyro is not the rotation in the desired plane.